In the aforementioned copending applications, a problem associated with high speed cutting of sheeting materials is overcome by effecting the cutting in the direction of movement of the sheeting material. Though this can solve a number of problems, there remains a special problem associated with the high speed cutting of relatively thin films of extensible materials, particularly the high speed cutting of relatively thin films of plastic materials such as thermoplastic films. The problem relates to the quality of the cut in the extensible material and the handling of discardable selvage. The term "relatively thin films" encompasses films having a thickness less than about 5 mils. Because extensible materials, by definition, can be stretched, they are particularly vulnerable when the cutting force has an unrestrained impact on the material. This can be visualized by the following:
If one holds a wide sheet of extensible film such as low density polyethylene at its edges but unrestrained in the interior portion, and then impacts a surface of the interior portion with a blunt instrument, the film will stretch in the direction of the impact. When the force of the impact is sufficient to cause a tear in the film, because the extensible material is sufficiently non-elastomeric so as not to regain its original shape, stretched portions of the film will reside at the tear and about the tear leaving a poor quality ragged cut in the material and a permanently deformed material in the area of the tear. As one reduces the bluntness of the instrument, less stretching occurs and the quality of the cut commensurately improves and permanent deformation is commensurately reduced. It is, however, not eliminated.
Waterjet cutting can be effected on such extensible materials. It can act as the aforementioned blunt instrument or as a less blunt instrument in cutting the material. Because it is a high pressure stream of water, it has the capacity of spattering, splashing, splaying, depending how it is used, and none of these three S's is favorable to its application as a cutting tool. For example, if one introduces a solid surface in its path, the stream will be deflected and subjected to all of the 3 S's. If the object being cut is an extensible thermoplastic film and is rested on a solid surface when it is waterjet cut, the waterjet stream will pound the solid surface after penetrating and cutting the film. The impact on the solid surface will cause the stream to be deflected in many directions, one of which will be back into the film being cut causing, as a minimum effect, movement of the film particularly about the area being cut and this forebodes distortions and unpredictable patterns in the cutting. This problem exists even if the solid surface is a thin wire, see Leslie, et al., infra. These kinds of problems are compounded when the cutting pattern moves lateral of a moving film and the line speed of the moving film is high, such as greater than 100 lineal feet per minute. They become even more severe problems when the waterjet cutter slices into and out of an edge of a film of an extensible thermoplastic material to cut out a part that is free to move from the film. If the cutting action throws water into the film, the cut out part can be tossed about the work area to potentially interfere with the cutting operation. If the line speed is high, viz. greater than 100 lineal feet per minute, air currents become an injected problem that can cause the cut part to move within and about the cutting operation thus introducing further potential interference with the cutting operation.
A problem associated with waterjet cutting is "splashback." It is dealt with by Leslie, et al, in U.S. Pat. No. 3,978,748, patented Sept. 7, 1976, according to the following:
The splashback problem is associated with the use of a solid table supporting the material during the cutting action. The patentees were not aware of the issue of extensibility or were unconcerned by it because it was not the problem which the patentees had solved. These patentees dealt with the problem of water removal but not the effective support of the material so as to avoid the extensibility problem. Moreover, the patentees fail to mention that as the water spray passes over a supporting wire, a certain level of splashback is inevitable though evidently that level would be less than the level of splashback generated on a solid flat table.
Leslie, et al. deal with the water removal problem by providing a water catcher in registration with the waterjet cutter such that wherever the waterjet moves, so does the water catcher. The water catcher is located below the wire trays to catch the water that passes through the material and the tray. The water catcher in Leslie, et al. is not part of the support of the material being cut nor is it in open registration with the support such that no part of the support intervenes the material being cut and the water catcher.
Waterjet cutting has been used to cut extensible materials where either the piece that is being cut is moved or the waterjet cutting means is moved. There are known processes where both are moved in linear or essentially linear directions. For example, the cutter is moved essentially laterally of the piece or the waterjet cutting means is oscillated within a small arc laterally into a linearly moving film of extensible material.
There are products formed from plastic film that are produced in continuous runs. There are situations when a cut through an edge of the film is required in order to form the desired product. Such typically generates a disposable selvage. In the case of Kloehn, et al., U.S. Pat. No. 4,567,796, patented Feb. 4, 1986, and U.S. Pat. No. 4,573,382, patented Mar. 4, 1986, this is accomplished by the use of oscillating cutting devices which fail to follow in the direction of the movement of the object being cut. The patentees use waterjet cutting to trim the edge of a continuous run of plastic sheet to make the leg openings of a baby diaper. The continuous run of plastic is supplied on a conveyer of undefined description. The patent's drawings show the conveyers as endless belts, apparently of solid construction (the equivalent of a solid table). The patentees were either unconcerned with splashback and extensibility or were operating the waterjet cutter in a manner, such as at slow line speed, that the problems of splashback and extensibility could be accepted. The application filing dates of these patents was May 7, 1984, thus the technology is reasonably presumed to represent the current state of the art in waterjet cutting of extensible materials in a continuous operating mode.
Porter, U.S. Pat. No. 4,335,636, patented Jun. 22, 1982, utilizes waterjet cutting to cut gypsum boards and catches the water in a trough below the suspended board above it. The patentee avoided splashback, but the device used is not practical for the waterjet cutting of an extensible material as described above.
Niedermeyer, U.S. Pat. No. 4,266,112, patented May 5, 1981, fails to suggest the breadth of materials that the patent's process is expected to cut. The patent deals with "webs" and "materials," and only at col. 9, lines 11-13, does the patentee express some definition for those terms, with the following:
Expanded polyurethane foam is formed typically of thick sheets greater than about 30 mils. Though flexible polyurethane foams are made of extensible materials, rigid polyurethane foams need not be. The fact that the foam is defined as plastic sheets is an insufficient clue as to the actual identity of the materials being referred to by the patentee. Many rigid polyurethane foams are heat shapeable and, therefore, are properly characterized as plastics even though they would not be classed as thermoplastics.
Niedermeyer fails to suggest how the water of the waterjet, after cutting, is removed from the proximity of the material being cut. At best, the patentee suggests that the water is deflected to a trough, suggesting that a trough is the collection device. In every illustration, the material being cut is unsupported at about the area where cutting is being effected.
Miyakita, et al., U.S. Pat. No. 4,048,885, is directed to the use of waterjet cutting "of a moving sheet material having a large thickness and a large width which is difficult to cut with the conventional rotary shear although not impossible" (see col. 1, lines 44-47). As with the preceding prior art, little concern is shown for supporting the material, the handling of the splashback problem, the extensibility of the material and/or the removal of water from the cutting site.
Reciprocation or oscillation of a waterjet cutter rapidly into and out of an edge of a moving sheet of material produces a slanted slit having a gentle arc until the apex portion which is an abrupt curve that generates a parabolic slice defining a sharply formed or narrow apex (or tip). If such techniques are employed to effect a parabolic cut through a folded edge of a plastic or paper sheet, the unfolded sheet will not be an ellipse, but rather two (2) parabolas joined to form a hole and each juncture is an angle of about 30 or greater. At maximum line speed, this type of reciprocating cutting motion will typically create a poor quality cut because the cutting speed exceeds the speed at which the cutting stream most effectively cuts the material and because of the dynamic loads imposed on the cutter in the course of rapid cutter reciprocation which causes splaying of the cutter means during the turnaround. Kloehn, et al., U.S. Pat. No. 4,567,796, patented Feb. 4, 1986, and U.S. Pat. No. 4,573,382, patented Mar. 4, 1986. In order to vary the kind of cut performed by such cutters, it is necessary to cause them to alter their motion during the cutting action. This introduces complications in the mechanics of their operation. U.S. Pat. No. 4,573,382 describes the oscillation of a cutter into the sheeting and with cam arrangements varying the cutter's motion within the sheeting to elongate the hole that is cut. Such a cutting operation can impart high dynamic loads on the nozzle of the cutter which imposes stress on the cam system controlling the nozzle's movement. According to the patent, at col. 4, lines 13 et seq., cam means are put under great stress when used in oscillator waterjet cutters and "these stresses seriously limit the speed at which the web 6 can be cut. . ." To "minimize," but not necessarily overcome the problem, a "compromise cutting line" for the fluid jet is followed. Such apparently results in a compromise in the achievable cutting patterns, exhibiting the limitations of a process that places undue stress on the apparatus.
As pointed out previously, Kloehn, et al. fail to define a method for removing the water from the waterjet cutter from the cutting site. Water jetted into a solid surface will impose a detrimental stress on the waterjet cutter by the dynamic load transmitted back into the water stream as it ricochets from the solid surface. Thus, not only do the patents' process admittedly have to deal with stresses from the dynamic loads due to the oscillation action, it has to deal with the stresses imparted by the pressures generated at the solid surfaces where the cutting is taking place. The waterjet cutting process of these patents further suffer from the splashback problem and deformation of the thermoplastic film by virtue of water splaying on the solid support surface.
In addition, Kloehn, et al. fail to recognize the serious corrosion and surface deterioration problems introduced by the repeated pounding of a solid support surface by waterjet spray.
It would be desirable to be able to effect a repetitive selvage cut in a continuous run of a thermoplastic film without having to slow or stop the run and without distorting the cut selvage edge of the film.
It would be desirable to effect a cut in a sheeting material which is not limited by stresses imposed on the cutting means because of dynamic loads imparted by water ricocheting from any solid surface.
It would be desirable to effect a repetitive selvage cut in a continuous run of a thermoplastic film or films without suffering from splashback problems and generating raggedly cut edges.
This invention is directed to a process and apparatus for making a selvage cut in thermoplastic sheeting material which avoids the disadvantages of the prior art.